Fuel formulations

ABSTRACT

A diesel fuel formulation containing (i) a lower molecular weight dialkyl carbonate (DAC) selected from dimethyl carbonate (DMC), diethyl carbonate (DEC) and mixtures thereof; (ii) di-n-butyl carbonate (DBC); and optionally (iii) an additional diesel fuel component is provided.

This application claims the benefit of European Application No.09176885.3 filed Nov. 24, 2009 which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates to diesel fuel formulations, their preparationand their use.

BACKGROUND TO THE INVENTION

In the interests of the environment, and to comply with increasinglystringent regulatory demands, it is necessary to increase the amount ofbiofuels used in automotive fuels.

Biofuels are combustible fuels, typically derived from biologicalsources, which result in a reduction in “well-to-wheels” (ie from sourceto combustion) greenhouse gas emissions. In diesel fuels for use incompression ignition engines, the most common biofuels are fatty acidalkyl esters (FAAEs), in particular fatty acid methyl esters (FAMEs)such as rapeseed methyl ester and palm oil methyl ester; these are usedin blends with conventional diesel fuel components.

Lower dialkyl carbonates, in particular dimethyl carbonate (DMC) anddiethyl carbonate (DEC), are also biofuels which have in the past beenadded to both gasoline and diesel fuels. They have been used forinstance as oxygenates, as combustion improvers and to reduce pollutionlevels. However, there are a number of practical constraints on theconcentrations at which DMC and DEC can be included in automotive dieselfuels. In particular, their low flash points—and the consequentlyreduced flash points of blends containing them—tend to limit DMCconcentrations to less than 2% v/v and DEC concentrations to around 3%v/v. As a result, dialkyl carbonates have received little attention asfuel components other than at relatively low levels.

FAMEs have much higher flash points than both DMC and DEC, and as aresult could potentially be blended with the dialkyl carbonates in orderto increase their suitability for use as diesel fuel components.However, there can be a number of drawbacks associated with the use ofFAMEs in diesel fuels, in particular at higher concentrations. Theaddition of a FAME to a diesel fuel formulation raises its cloud point,to an extent dependent on the FAME concentration. It also raises thecold filter plugging point (CFPP) of the formulation. Moreover, due tothe incomplete esterification of oils (triglycerides) during theirmanufacture, FAMEs can contain trace amounts of glycerides, inparticular monoglycerides. These glycerides tend, on cooling, tocrystallise out before the FAMEs themselves, and can cause fuel filterblockages. These three effects can compromise the cold weatherperformance of a FAME-containing diesel fuel. It can therefore bedifficult to formulate diesel fuel/FAME blends within the relevantregulatory specifications, particularly in colder climates.

FAMEs and their oxidation products also tend to accumulate in engineoil; this too has limited their use in modern FAME/diesel blends. Athigher concentrations they can also cause fouling of fuel injectors.FAMEs are also more expensive to produce than ethanol (the biofuel mostcommonly included in gasoline formulations), and their world productionlevels much lower.

It would be desirable to provide new biofuel-containing diesel fuelformulations which could overcome or at least mitigate the aboveproblems.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention there is provided adiesel fuel formulation containing (i) a lower molecular weight dialkylcarbonate (DAC) selected from dimethyl carbonate (DMC), diethylcarbonate (DEC) and mixtures thereof; and (ii) di-n-butyl carbonate(DBC).

The formulation may also contain (iii) an additional diesel fuelcomponent.

In another embodiment, a process is provided for the preparation of adiesel fuel formulation comprising blending together (i) a lowermolecular weight DAC selected from DMC, DEC and mixtures thereof; and(ii) DBC.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that DBC can raise the flash points of diesel fuelformulations containing DMC or DEC, to a greater extent than might havebeen predicted based on its own flash point of 89° C. In particular, itsflash point raising effect appears to be significantly greater, at anygiven concentration, than that of the same concentration of the commonbiodiesel component rapeseed methyl ester (RME), despite the fact thatthe flash point of neat RME (170° C.) is far higher than that of DBC.

This discovery allows DBC to be used as a component of DMC- orDEC-containing diesel fuel formulations, for the purpose of raisingtheir flash points, but without the drawbacks potentially associatedwith the inclusion of a FAME for the same purpose. In turn, diesel fuelscan be formulated with higher concentrations of DMC and DEC, without orwithout undue impact on their overall flash points. The inclusion of theDBC can help to bring a DMC- or DEC-containing diesel fuel within adesired flash point specification; this can be of particular value wherethe fuel is for use in a warmer climate. Thus the present invention isable to provide more optimised methods for formulatingbiofuel-containing diesel fuel formulations, in particular summer gradediesel fuels, more particularly to achieve target flash points.

There can be a number of advantages to increasing the concentration ofDACs in a diesel fuel formulation. Not only does DBC have a lower cloudpoint (−60° C.) and CFPP (−36° C.) than FAMEs, and thus the ability tolower the cloud point and CFPP of a fuel formulation to which it isadded; it also has low toxicity and is biodegradable. Its inclusion in adiesel fuel formulation can increase the total bioenergy content of theformulation, so long as the DBC is derived from a biological source, andin turn reduce the greenhouse gas emissions associated with theproduction and use of the fuel, yet with fewer of the drawbacksassociated with higher FAME concentrations. DBC can also be producedfrom renewable ingredients (carbon dioxide and bio-butanol). When usedin diesel fuels, it can be a cheaper alternative to the moretraditionally used FAME biofuel components such as RME.

In an embodiment of the invention, the lower molecular weight DAC usedin the formulation is DMC. In an embodiment, it is DEC.

The dialkyl carbonates used in a fuel formulation according to theinvention (DMC and/or DEC, and DBC) may be obtained from any knownsource, of which many are available. They can for example be synthesisedfrom the corresponding alcohol(s): methanol may be used as a startingmaterial for the production of DMC, ethanol for the production of DEC,and butanol for the production of DBC. Such alcohols may themselves bederived from biological sources.

DACs can also be prepared by oxidative carbonylation of alcohols, or bytransesterification of dimethyl carbonate with alcohols, or they may begenerated as co-products in the synthesis of monoethylene glycol fromethylene oxide and carbon dioxide via ethylene carbonate.

In an embodiment, it may be preferred for the DACs not to have beensynthesised using phosgene (COCl₂), as this may introduce undesirableimpurities such as chlorides or carbonochloridic acid derivatives. Suchimpurities may contribute to deposit, stability and corrosion problemsin a fuel formulation.

The concentration of the DMC, DEC or mixture thereof, in a diesel fuelformulation according to the invention, may be 0.5% v/v or greater, or 1or 2 or 3% v/v or greater, or in cases 3.5 or 4 or 4.5 or 5% v/v orgreater. Its concentration may be up to 20% v/v, or up to 15 or 12 or10% v/v.

The concentration of the DBC in the formulation may be 0.5% v/v orgreater, or 1 or 2 or 3% v/v or greater, or in cases 3.5 or 4 or 4.5 or5% v/v or greater. Its concentration may be up to 99.5 or 99 or 98% v/v,or up to 95 or 90% v/v, or up to 80 or 70 or 60 or 50 or 30 or 25 or 20%v/v, or up to 15 or 12 or 10% v/v.

The volume ratio of the lower molecular weight DAC (i) to the DBC (ii)in the formulation may for instance be up to 25:1, or up to 10:1 or 5:1or 2:1. The ratio may be 1:199 or greater, or 1:99 or greater, or 1:90or greater, or 1:75 or greater, or 1:50 or greater. It may be 1:25 orgreater, or 1:10 or 1:5 or 1:2 or greater. It may be 1:1 orapproximately 1:1.

The additional diesel fuel component (iii), if present, may be any fuelcomponent suitable for use in a diesel fuel formulation and thereforefor combustion within a compression ignition (diesel) engine. It willtypically be a liquid hydrocarbon middle distillate fuel, more typicallya gas oil. It may be petroleum derived. It may be or contain a kerosenefuel component.

Alternatively it may be synthetic: for instance it may be the product ofa Fischer-Tropsch condensation. It may be derived from a biologicalsource. It may be or include an oxygenate such as an alcohol (inparticular a C1 to C4 or C1 to C3 aliphatic alcohol, more particularlyethanol).

An additional fuel component (iii) will typically boil in the range from150 or 180 to 360° C. (ASTM D86 or EN ISO 3405). It will suitably have ameasured cetane number (ASTM D613) of from 40 to 70 or from 40 to 65 orfrom 51 to 65 or 70.

In one embodiment, a formulation according to the invention may containa mixture of two or more additional diesel fuel components (iii).

The concentration of the component(s) (iii) in the formulation, ifpresent, may be 2 or 5 or 10% v/v or greater, or 20 or 30 or 40% v/v orgreater. In embodiments of the invention, it may be 50 or 60 or 70% v/vor greater, or 75 or 80 or 85% v/v or greater, or 90 or 92 or 95% v/v orgreater. It may be up to 98% v/v, or up to 95 or 92 or 90 or 85 or 80%v/v. In cases it may be up to 70 or 60 or 50% v/v. The component(s)(iii) may represent the major part of the fuel formulation: afterinclusion of the lower molecular weight DAC (i), the DBC (ii) and anyoptional fuel additives, the component(s) (iii) may therefore representthe balance to 100%. Alternatively, the fuel formulation may comprisethe lower molecular weight DAC and the DBC, optionally with one or morediesel fuel additives, but without any additional diesel fuelcomponents.

A fuel formulation according to the invention suitably has a flash point(ASTM D92 or D93, or IP 34) of 38° C. or higher, or of 40 or 45° C. orhigher, or of 50 or 55 or in cases 60 or 65 or 70 or 75° C. or higher.

The formulation of the invention should be suitable for use in acompression ignition (diesel) internal combustion engine. Such an enginemay be either heavy or light duty. The formulation may in particular besuitable for use as an automotive diesel fuel.

In an embodiment, the formulation is suitable and/or adapted for use asa “summer grade” automotive diesel fuel, for use in warmer climates suchas Australasia and/or in warmer seasons. In an embodiment, it issuitable and/or adapted for high temperature use such as at 30° C. orhigher.

In further embodiments, the formulation may be suitable and/or adaptedfor use as an industrial gas oil, or as a domestic heating oil.

The formulation will suitably comply with applicable current standarddiesel fuel specification(s) such as for example EN 590 (for Europe) orASTM D975 (for the USA). By way of example, the overall formulation mayhave a density from 820 to 845 kg/m³ at 15° C. (ASTM D4052 or EN ISO3675); a T95 boiling point (ASTM D86 or EN ISO 3405) of 360° C. or less;a measured cetane number (ASTM D613) of 51 or greater; a kinematicviscosity at 40° C. (ASTM D445 or EN ISO 3104) from 2 to 4.5centistokes; a sulphur content (ASTM D2622 or EN ISO 20846) of 50 mg/kgor less; and/or a polycyclic aromatic hydrocarbons (PAH) content (IP391(mod)) of less than 11% w/w. Relevant specifications may howeverdiffer from country to country and from year to year, and may depend onthe intended use of the formulation. Moreover a formulation according tothe invention may contain fuel components with properties outside ofthese ranges, since the properties of an overall blend may differ, oftensignificantly, from those of its individual constituents.

The relative concentrations of the components (i) to (iii) may be chosento achieve desired properties for the formulation as a whole, forexample a minimum desired flash point. Thus the relative concentrationswill also depend on the physicochemical properties of the individualcomponents. Suitable concentrations may be calculated by applyingappropriate blending rules to the properties (in particular the flashpoints, but also potentially other properties such as cloud pointsand/or cetane numbers) of the individual components, and may bevisualised using a two- or three-way composition plot.

A fuel formulation according to the invention may contain standard fuelor refinery additives which are suitable for use in diesel fuels. Manysuch additives are known and commercially available.

According to another embodiment of the present invention, there isprovided a process for the preparation of a diesel fuel formulation,which process involves blending together (i) a lower molecular weightDAC selected from DMC, DEC and mixtures thereof and (ii) DBC, optionallywith (iii) one or more additional diesel fuel components, and optionallywith one or more fuel additives. The process may be used to produce atleast 1,000 litres of the fuel formulation, or at least 5,000 or 10,000or 25,000 litres, or at least 50,000 or 75,000 or 100,000 litres.

In an embodiment, the lower molecular weight DAC (i) and the DBC arepremixed in an appropriate volume ratio, and if necessary the mixturethen blended with one or more additional fuel components (iii). The DACmixture may for instance be blended with the component(s) (iii) at aconcentration of up to 30% v/v based on the product fuel formulation, orat a concentration of up to 25 or 20% v/v, or up to 15 or 10% v/v. Itmay be blended at a concentration of 1% v/v or greater based on theproduct formulation, or of 2 or 3 or 4 or 5% v/v or greater, or in casesof 6 or 7 or 8 or 9 or 10% v/v or greater. Mixing the DBC with the lowermolecular weight DAC can, by raising its flash point, help to improveits handling and storage properties.

In another embodiment of the invention provides a method of operating aninternal combustion engine, and/or a vehicle which is driven by aninternal combustion engine, which method involves introducing into acombustion chamber of the engine a diesel fuel formulation according tothe first aspect of the invention. The engine is suitably a compressionignition (diesel) engine. Such a diesel engine may be of the directinjection type, for example of the rotary pump, in-line pump, unit pump,electronic unit injector or common rail type, or of the indirectinjection type. It may be a heavy or a light duty diesel engine.

In another embodiment also embraces introducing DBC into a reservoirwhich contains a DMC- and/or DEC-containing diesel fuel formulation,prior to introduction of the resultant mixture into a combustion chamberof the engine. In other words, the diesel fuel formulation of theinvention may be prepared in situ in a reservoir from which fuel is fedinto an internal combustion engine.

According to another embodiment of the invention there is provided theuse of DBC, in a diesel fuel formulation containing a lower molecularweight DAC selected from DMC, DEC and mixtures thereof, for the purposeof increasing the flash point of the formulation.

The flash point of a fuel formulation is the lowest temperature atwhich, under a predetermined set of test conditions, the application ofan ignition source causes the vapour above a sample of the formulationto ignite and the flame to propagate across the surface of the liquid.It can be measured using a standard test method such as ASTM D92 or D93,IP 34, or an analogous method: a suitable procedure is described inExample 1 below.

The invention may be used to achieve any degree of increase in the flashpoint of the formulation. It may be used for the purpose of achieving aflash point at or above a desired target value.

By way of example, the invention may be used to increase the flash pointof the formulation by at least 0.2% of its value (expressed in Kelvin)prior to addition of the DBC, or by at least 0.5 or 0.6%, or by at least0.8 or 1% or in cases even by 2 or 5 or 8 or 10% or more.

The diesel fuel formulation may contain one or more additional dieselfuel components (iii) in addition to the lower molecular weight DAC. Itmay in particular be a summer grade diesel fuel formulation. It willtypically have a flash point, prior to addition of the DBC, which islower than that of the DBC alone (i.e. which is lower than 89° C.)

It has been found that DBC can “boost” the flash point of a diesel fuelformulation containing DMC and/or DEC, above the level that would beexpected if conventional blending rules applied. This phenomenon has anumber of potential uses. Firstly, it can allow the achievement of ahigher flash point than was previously thought possible, for any givenconcentration of DBC, in a DMC- and/or DEC-containing diesel fuelformulation. Secondly, it can allow the achievement of a target flashpoint using a lower than predicted concentration of DBC. This in turncan reduce the costs which might be associated with the addition of DBCto the formulation. Thirdly, it can allow the use of a higher DMC and/orDEC concentration than would have been predicted to be feasible, whilststill maintaining the flash point of the overall formulation at or abovea desired target value; this in turn can boost the bioenergy content ofthe formulation, as well as increasing the advantages associated withthe DMC and/or DEC, for example reductions in cloud point and CFPP.Fourthly, the DBC may be used to replace, at least partially, a FAAEwhich would otherwise have been included in the formulation in order toincrease its flash point.

There is provided a method for increasing the flash point of a dieselfuel formulation which contains a lower molecular weight DAC selectedfrom DMC, DEC and mixtures thereof, in order to achieve a target minimumflash point X, which method comprises adding to the formulation aconcentration c of DBC, wherein c is lower than the minimumconcentration c′ of DBC which theory would predict needed to be added tothe formulation in order to achieve flash point X.

The theoretical DBC concentration, c′, may be calculated using anysuitable flash point blending rule. It may for instance be calculated asfollows, making use of the Wickey-Chittenden flash point model.

The flash point index of a blend of fuel components (in this case of thelower molecular weight DAC, the DBC and any additional diesel fuelcomponents present) is calculated by combining the flash point indicesof the blend components as a function of their volume fractions in theblend:

${Index}_{blend} = {\sum\limits_{i}^{\;}\; {V_{i}{Index}_{i}}}$

where V_(i) is the volume fraction of component i and Index_(i) is theflash point index for component i.

The flash point index for each component can be calculated using theWickey-Chittenden model (Wickey R. O. and Chittenden D. H., HydrocarbonProcessing, 42(6), 1963: 157-158):

$\begin{matrix}{{{Log}_{10}\left( {Index}_{i} \right)} = {{- 6.1188} + \frac{2414}{{FP} + 230.5556}}} & \left( {{equation}\mspace{14mu} 1} \right)\end{matrix}$

where FP is the flash point of the component.

When using equation (1), the blending flash point of DMC may be taken tobe 0° C., and that of DEC 17.7° C. These figures take account ofpotential mismatches in Hansen solubility parameters between the DAC andany hydrocarbon fuel components present.

Having calculated the flash point index for the overall blend, theWickey-Chittenden equation (1) may be used to calculate back the flashpoint for the blend. By inserting suitable values into these equations,it is possible to work back from a target flash point X to determine thevolume fraction or concentration c′ (of DBC) which would be required toachieve X.

In an embodiment, the actual DBC concentration c may be at least 0.5%v/v lower than the predicted concentration c′, or at least 1 or 2 or 5%v/v lower, or in cases at least 10 or 25 or 50 or 75% v/v lower.

In another embodiment, DBC is used at a concentration c, in a dieselfuel formulation which contains a lower molecular weight DAC selectedfrom DMC, DEC and mixtures thereof, for the purpose of increasing theflash point of the formulation by an amount x, wherein x is greater thanthe flash point increase x′ which theory would predict would result fromadding DBC to the formulation at concentration c.

The theoretical (predicted) flash point increase x′ may be calculatedusing the equations above. The actual flash point increase x may forinstance be at least 0.5° C. higher than the predicted increase x′, orat least 1 or 1.5 or 2° C. higher, or in cases at least 5 or 8 or 10° C.higher.

There is provided the use of DBC, at a concentration c, in a diesel fuelformulation which contains a lower molecular weight DAC selected fromDMC, DEC and mixtures thereof, for the dual purposes of:

-   a) achieving a target minimum flash point X for the formulation; and-   b) allowing an increase in the concentration of the lower molecular    weight DAC to a level above the maximum concentration d′ which    theory would predict could be included in the formulation, after    addition of the DBC at concentration c, without reducing the flash    point of the formulation below the target minimum X.

Again, the theoretical maximum concentration d′, for the lower molecularweight DAC, may be calculated using the above rules.

It may be desirable to include DMC and/or DEC in a diesel fuelformulation for a number of reasons, for example to improve the coldflow properties of the formulation (in particular to reduce its cloudpoint and/or CFPP), and/or to reduce emissions from a fuel-consumingsystem (typically an engine) running on the formulation, and/or toreduce greenhouse gas emissions associated with the production and useof the formulation, and/or to increase the bioenergy content of theformulation, and/or as a combustion improver. However it has beennecessary, in the past, to balance such benefits against the generallyundesirable reduction in flash point which results from increasing theconcentration of the DMC and/or DEC. The ability to increase theirconcentration without undue detriment to the flash point of theformulation can therefore provide significant advantages. Generallyspeaking the present invention can provide greater flexibility in fuelformulation, allowing a target flash point to be achieved more readilyby altering the concentration of the added DBC.

DBC may be used in a diesel fuel formulation containing a lowermolecular weight DAC selected from DMC, DEC and mixtures thereof, forthe purpose of replacing, at least partially, a fatty acid alkyl ester(FAAE) which is or would otherwise have been included in theformulation.

The FAAE may in particular be a fatty acid methyl ester (FAME). It mayfor instance be RME. It may have been included, or intended to beincluded, at least partly for the purpose of increasing the flash pointof the DMC- and/or DEC-containing formulation. According to the eighthaspect of the invention, the DBC may again be used at a lowerconcentration than theory would predict to be necessary in order toachieve a target flash point after reduction of the amount of the FAAEin the formulation. Thus, the DBC may be used to achieve a greaterreduction in the FAAE concentration (including in cases reduction tozero) than theory would predict to be possible whilst still achievingthe target flash point.

In the context of the present invention, “use” of DBC in a diesel fuelformulation means incorporating the DBC into the formulation, typicallyas a blend (ie a physical mixture) with one or more other diesel fuelcomponents. The DBC will conveniently be incorporated before theformulation is introduced into an engine or other system which is to berun on the formulation. Instead or in addition the use of DBC mayinvolve running a fuel-consuming system, typically an internalcombustion engine, on a diesel fuel formulation containing the DBC,typically by introducing the formulation into a combustion chamber of anengine.

In the context of the invention, “achieving” a desired target propertyalso embraces—and in an embodiment involves—improving on the relevanttarget. Thus for instance the DBC may be used to produce a fuelformulation which has a flash point above a desired target value.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, mean “including but not limited to”, anddo not exclude other moieties, additives, components, integers or steps.Moreover the singular encompasses the plural unless the contextotherwise requires: in particular, where the indefinite article is used,the specification is to be understood as contemplating plurality as wellas singularity, unless the context requires otherwise.

Preferred features of each aspect of the invention may be as describedin connection with any of the other aspects. Other features of theinvention will become apparent from the following examples. Generallyspeaking the invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims and drawings). Thus features, integers,characteristics, compounds, chemical moieties or groups described inconjunction with a particular aspect, embodiment or example of theinvention are to be understood to be applicable to any other aspect,embodiment or example described herein unless incompatible therewith.Moreover unless stated otherwise, any feature disclosed herein may bereplaced by an alternative feature serving the same or a similarpurpose.

The present invention will now be further described with reference tothe following non-limiting examples.

Example 1

Diesel fuel formulations were prepared by blending one or more ofdiethyl carbonate (DEC), di-n-butyl carbonate (DBC) and rapeseed methylester (RME) with a diesel base fuel DBF1. The DEC, DBC and RME were eachadded to the formulations at a concentration of 5% v/v.

The base fuel was a commercially available zero sulphur automotivediesel base fuel, ex. Shell. It had a flash point (IP 34) of 72.5° C. Ithad a density at 15° C. (ASTM D4052) of 840.9 kg/m³, an initial boilingpoint (ASTM D86) of 177.5° C., a T95 boiling point (ASTM D86) of 357°C., a final boiling point (ASTM D86) of 363° C., a measured cetanenumber (ASTM D613) of 55.4, an E250 (IP 123) of 22.4° C. and an E350 (IP123) of 89.7° C.

The DEC and DBC were sourced from Sigma Aldrich, UK and the RME fromADM.

The flash points of the prepared formulations were measured by thePensky-Martens Closed Cup method (IP 34), as follows. A sample of theformulation under test was placed in the test cup of a Pensky-Martensapparatus and heated to give a constant rate of temperature increasewith continuous stirring. An ignition source was directed through anopening in the test cup lid at regular temperature intervals, withsimultaneous interruption of stirring. The lowest temperature at whichthe application of the ignition source caused the vapour of the sampleto ignite and the flame to propagate over the surface of the liquid wasrecorded as the flash point at the ambient barometric pressure. Thisvalue was corrected to standard atmospheric pressure as per IP 34.

Three readings were taken for each formulation, in order to calculate anaverage (mean) flash point. The results are shown in Table 1 below,together with the flash points for the neat DEC, DBC and RME, which wereobtained using the standard test method IP 34.

Table 1 also shows theoretical flash points for the formulations,calculated using the Wickey-Chittenden equation above using a blendingflash point of 17.7° C. for the DEC.

TABLE 1 Predicted Measured Measured Average flash improvementimprovement flash point by relative to relative to % v/v % v/v % v/vpoint W-C 5% v/v DEC that predicted DEC RME DBC (° C.) (° C.) (° C.) byW-C (° C.) 0 0 0 72.5 — — — 5 0 0 51 51.9 — 0.9 5 5 0 53 52.0 +2 +1.0 50 5 54.5 52.0 +3.5 +2.5 100 0 0 25 — — — 0 100 0 170 — — — 0 0 100 89 —— —

It can be seen from Table 1 that the base fuel alone, with no dialkylcarbonate or FAME present, has an average flash point of 72.5° C. Theaddition of 5% v/v DEC (neat flash point only 25° C.) reduces thisconsiderably, to 51° C.

RME has a neat flash point of 170° C., far higher than that of the basefuel. Yet the addition of 5% v/v RME to the base fuel/DEC blend raisesits flash point by only 2° C. In contrast, the addition of 5% v/v DBC tothe base fuel/DEC blend, in accordance with the invention, raises theaverage flash point by 3.5° C., to 54.5° C. This increase isparticularly surprising since the flash point for neat DBC is 89° C.,much lower than that of neat RME. It would therefore have been expectedthat RME would have a greater effect on the flash point of a DEC/basefuel blend than would the same volume of DBC. As can be seen from thefifth column of Table 1, the increase in flash point caused by theaddition of 5% v/v DBC to the DEC/base fuel blend is in fact greaterthan the Wickey-Chittenden model would have predicted.

These results suggest that there is a synergistic interaction betweenthe DEC and the DBC, which is not present between DEC and RME.

Example 2

Example 1 was repeated, but adding the DEC, DBC and RME to a seconddiesel base fuel, DBF2, at 10% v/v. DBF2 was a commercially availablezero sulphur automotive diesel base fuel, ex. Shell. It had a flashpoint (IP 34) of 65° C. It had a density at 15° C. (ASTM D4052) of 833kg/m³, an initial boiling point (ASTM D86) of 165° C., a T95 boilingpoint (ASTM D86) of 340° C., a final boiling point (ASTM D86) of 352°C., a measured cetane number (ASTM D613) of 54.1, an E250 (IP 123) of28.2° C. and an E350 (IP 123) of 96.8° C.

The flash point results are shown in Table 2 below. Again, the tableshows predicted flash points as calculated using the Wickey-Chittendenequation, using a DEC blending flash point of 17.7° C.

TABLE 2 Predicted Measured Measured Average flash improvementimprovement flash point by relative to relative to % v/v % v/v % v/vpoint W-C 10% v/v that predicted DEC RME DBC (° C.) (° C.) DEC (° C.) byW-C (° C.) 0 0 0 65 — — 10 0 0 44 44.2 — 0.2 10 10 0 45 44.4 +1 0.6 10 010 46 44.3 +2 +1.7

Here the addition of 10% v/v DEC reduces the flash point of the basefuel from 65 to 44° C. Adding 10% v/v RME to the DEC/base fuel blendincreases its flash point by only 1° C., whereas adding 10% v/v DBC tothe blend increases the flash point by 2° C. Again, the DBC unexpectedlyhas a greater impact on the blend flash point than does the RME, despitethe fact that neat DBC has a much lower flash point than neat RME. TheDBC also increases the blend flash point by more than would have beenpredicted using the Wickey-Chittenden model.

Example 3

Example 1 was repeated, but using dimethyl carbonate (DMC) instead ofDEC, and the base fuel DBF2 as in Example 2. The DMC was sourced fromSigma Aldrich, UK.

The flash point results are shown in Table 3 below. The fifth columnshows predicted flash points as calculated using the Wickey-Chittendenequation, using a blending flash point of 0° C. for the DMC.

TABLE 3 Predicted Measured Measured Average flash improvementimprovement flash point by relative to relative to % v/v % v/v % v/vpoint W-C 5% v/v that predicted DEC RME DBC (° C.) (° C.) DMC (° C.) byW-C (° C.) 0 0 0 65 — — — 5 0 0 30 31.5 — 1.5 5 5 0 31 31.6 +1 0.6 5 0 566 31.6 +36 +34.4 10 0 0 16.5 — — —

DBC can be seen to have an even greater effect on the flash point of aDMC/base fuel blend than on a DEC/base fuel blend. Again, the additionof 5% v/v of DBC to a blend of 5% v/v DMC in the diesel base fuel causesa far greater increase in the blend flash point than does the additionof 5% v/v RME, despite the higher flash point of neat RME. This increaseis also significantly greater than the Wickey-Chittenden model wouldhave predicted. Again there appears to be a synergistic interactionbetween the DMC and the DBC as regards their combined flash points.

Example 4

Example 1 was repeated, but using dimethyl carbonate (DMC) instead ofDEC, and the base fuel DBF3. DBF3 was a commercially available zerosulphur automotive diesel base fuel, ex. Shell. It had a flash point (IP34) of 65° C. It had a density at 15° C. (ASTM D4052) of 825 kg/m³, aninitial boiling point (ASTM D86) of 172° C., a T95 boiling point (ASTMD86) of 329° C., a final boiling point (ASTM D86) of 342° C., a measuredcetane number (ASTM D613) of 53.8, an E250 (IP 123) of 57.5° C. and noE350 (IP 123). The DMC was sourced from Sigma Aldrich, UK.

The flash point results are shown in Table 4 below. The fifth columnshows predicted flash points as calculated using the Wickey-Chittendenequation, using a blending flash point of 0° C. for the DMC and 89° C.for the DBC.

TABLE 4 Measured Predicted improvement flash point relative to % v/v %v/v Average flash by W-C that predicted DMC DBC point (° C.) (° C.) byW-C (° C.) 0 0 65 10 0 19 23.8 −4.8 10 10 26 23.8 2.2 15 0 17.5 19.4−1.9 15 10 25.5 19.4 6.1 20 0 18.5 16.3 2.2 20 10 23.5 16.3 7.2 20 2025.5 16.3 9.2

DBC can be seen to have an even greater effect on the flash point of aDMC/base fuel blend than on a DEC/base fuel blend. This increase is alsosignificantly greater than the Wickey-Chittenden model would havepredicted. Again there appears to be a synergistic interaction betweenthe DMC and the DBC as regards their combined flash points.

Example 5

Example 1 was repeated, but using diethyl carbonate (DEC) instead ofDMC, and the base fuel DBF3 from Example 4. The DMC was sourced fromSigma Aldrich, UK.

The flash point results are shown in Table 5 below. The fifth columnshows predicted flash points as calculated using the Wickey-Chittendenequation, using a blending flash point of 17.7° C. for the DEC and 89°C. for the DBC.

TABLE 5 Measured Predicted improvement flash point relative to % v/v %v/v Average flash by W-C that predicted DEC DBC point (° C.) (° C.) byW-C(° C.) 0 0 65 10 0 42 43.1 −1.1 10 10 49.5 43.4 6.1 15 0 38.5 38.8−0.3 15 10 45.5 39.0 6.5 20 0 36 35.6 0.4 20 10 43.5 35.7 7.8 20 20 45.535.9 9.6

This increase is also significantly greater than the Wickey-Chittendenmodel would have predicted. Again there appears to be a synergisticinteraction between the DEC and the DBC as regards their combined flashpoints.

1. A diesel fuel formulation comprising (i) a lower molecular weightdialkyl carbonate (DAC) selected from dimethyl carbonate (DMC), diethylcarbonate (DEC) and mixtures thereof; and (ii) di-n-butyl carbonate(DBC).
 2. The fuel formulation of claim 1 further comprising (iii) anadditional diesel fuel component.
 3. The fuel formulation of claim 1wherein the lower molecular weight DAC is DEC.
 4. The fuel formulationof claim 2 wherein the lower molecular weight DAC is DEC.
 5. The fuelformulation of claim 1 wherein the concentration of the lower molecularweight DAC is from 0.5 to 20% v/v.
 6. The fuel formulation of claim 2wherein the concentration of the lower molecular weight DAC is from 0.5to 20% v/v.
 7. The fuel formulation of claim 1 wherein the concentrationof the DBC is from 0.5 to 99.5% v/v.
 8. The fuel formulation of claim 1wherein the volume ratio of the lower molecular weight DAC to the DBC isapproximately 1:1.
 9. The fuel formulation claim 1 wherein the flashpoint of the formulation (ASTM D92 or D93) is 50° C. or higher.
 10. Thefuel formulation claim 2 wherein the flash point of the formulation(ASTM D92 or D93) is 50° C. or higher.
 11. A process for the preparationof a diesel fuel formulation comprising blending together (i) a lowermolecular weight DAC selected from DMC, DEC and mixtures thereof; and(ii) DBC.
 12. The process of claim 11 wherein at least one additionaldiesel fuel component is blended.
 13. A method of operating an internalcombustion engine, and/or a vehicle which is driven by an internalcombustion engine comprising introducing into a combustion chamber ofthe engine a diesel fuel formulation of claim
 1. 14. A method ofoperating an internal combustion engine, and/or a vehicle which isdriven by an internal combustion engine comprising introducing into acombustion chamber of the engine a diesel fuel formulation of claim 2.15. A method of operating an internal combustion engine, and/or avehicle which is driven by an internal combustion engine comprisingintroducing into a combustion chamber of the engine a diesel fuelformulation of claim
 5. 16. A method of operating an internal combustionengine, and/or a vehicle which is driven by an internal combustionengine comprising introducing into a combustion chamber of the engine adiesel fuel formulation of claim
 7. 17. A method of operating aninternal combustion engine, and/or a vehicle which is driven by aninternal combustion engine comprising introducing into a combustionchamber of the engine a diesel fuel formulation of claim
 8. 18. A methodfor increasing the flash point of a diesel fuel formulation whichcontains a lower molecular weight DAC selected from DMC, DEC andmixtures thereof, comprising adding to the formulation a concentration cof DBC, wherein c is lower than the minimum concentration c′ of DBCwhich theory would predict needed to be added to the formulation inorder to achieve flash point X.